https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202003261

Bifunctional Heterostructured Transition Metal Phosphides for Efficient Electrochemical Water Splitting

Electronic structure and d-band center control engineering over M-doped CoP (M = Ni, Mn, Fe) hollow polyhedron frames for boosting hydrogen production

Photo-chemical conversion of CO2 into solar fuels by photocatalysts has attracted significant attention. However, poor reaction efficiency remains a huge obstacle. Deep insight into the reaction mechanism of CO2, especially the active site of photocatalyst could provide scientific basis for the development of more efficient photocatalyst. The high inertness of CO2 and the multi-electron reduction feature on a photocatalyst determine high complexity of the reaction for the study. Here, pure Bismuth oxyhalides (BiOX, where X = F, CI, Br, I) with the layered structure, which were synthesized by both hydrothermal method and chemical precipitation method, were selected as model photocatalysts. The photocatalytic behaviors of the samples were evaluated by the CO2 reduction with H2O without the additional photosensitizer and sacrificial agent. The as-prepared BiOBr was observed to exhibit the best CO2 photoreduction performance under the simulated sunlight. The evolution rates of CO and CH4 are 21.6 μmol g−1 h−1 and 1.2 μmol g−1 h−1, respectively. The effects of water dosage, light intensity and irradiation time on the efficiency of CO2 photoreduction were investigated systematically. Interestingly, the reduction selectivity of CO2 to CO almost reaches 100% in the case of high light intensity. By combination with isotopic tracing method, electron spin-paramagnetic resonance (ESR), in-situ Fourier transform infrared (FTIR) characterization, positron annihilation lifetime (PAL) spectra, and Density functional theory (DFT) calculation, the oxygen vacancy mediated mechanism of photoreduction CO2 was suggested for BiOX. This work provides new information and insights to deepen the understanding for defect photocatalysis on CO2 reduction of semiconductor.

Photocatalytic reduction of CO2 on BiOX： Effect of halogen element type and surface oxygen vacancy mediated mechanism

A review of nanocarbons in energy electrocatalysis: Multifunctional substrates and highly active sites

Highly efficient, low-cost electrocatalysts having superior activity and stability are crucial for practical electrochemical water splitting, which involves hydrogen and oxygen evolution reactions (HER and OER). The sustainable production of hydrogen fuel from electrochemical water splitting requires the development of a highly efficient and stable electrocatalyst with low overpotential that drives electrochemical redox reactions. Electrochemical water splitting using highly active nickel-iron layered double hydroxide (NiFe LDH) catalyst having a very high turnover frequency and mass activity is considered as a potential contender in the area of electrocatalysis owing to the practical challenges including high efficiency and long durability at low overpotential, which shows great potential in future hydrogen economy. This review includes certain recommendations on enhancing the electrocatalytic performance of NiFe LDH-based electrocatalyst, particularly through morphology engineering, construction of hierarchical/core–shell nanostructures, and doping of heteroatoms through combined experimental assessment and theoretical investigations, which in turn improve the electrocatalytic performance. Finally, emphasis is made on the bifunctional activity of the NiFe LDH catalyst for overall water splitting. At the end, the conclusions and future outlook for the design of the NiFe LDH catalyst towards scale-up for their use as electrolyzer at the industrial level are also discussed.

Recent advances in highly active nanostructured NiFe LDH catalyst for electrochemical water splitting

With continuous research on photocatalytic water splitting, searching for efficient catalyst for hydrogen evolution reaction (HER) becomes popular topic in addition to main catalyst research. Transition metal phosphides are receiving intense attention due to its abundance in the Earth’s crust and comparable catalytic properties to noble metals. In this review, the synthesis approaches, HER reaction mechanism, photocatalytic activity, approaches to improve the activity of transition metal phosphides were reviewed and discussed. It was showed that the transition metal phosphides have great potential to reduce the cost of photocatalyst and promising application on water splitting. The stability problem and participation of poisonous reactant and product in its synthesis reaction limit its application and developing in a certain extent, but with the continuous efforts on the development and improvement of the synthesis methods, transition metal phosphides will find wide application in water splitting.

Synthesis and application of transition metal phosphides as electrocatalyst for water splitting

Bismuth enrichment as a common issue at the surface of bismuth vanadate (BiVO4) has been frequently reported as a severe limitation to its water oxidation kinetics. To address this problem, this study reports a novel approach to eliminate the surface bismuth enrichment by forming BiVO4/BiOCl heterojunction via surface hydrochloric acid treatment. Benefiting from type II band alignment of the BiVO4/BiOCl heterojunction, the treated photoanode delivers excellent PEC performance with a photocurrent density of 1.83 mA/cm2 at 1.23 V (vs. Ag/AgCl), compared to 1.27 mA/cm2 of untreated BiVO4 photoanode. This improvement could be attributed to the effective charge separation in heterojunction interface as the charge separation efficiency of BiVO4 increased from 28% to 41% at 1.23 V (vs. Ag/AgCl) after formation of BiVO4/BiOCl heterojunction.

Turning the unwanted surface bismuth enrichment to favourable BiVO4/BiOCl heterojunction for enhanced photoelectrochemical performance

WO3 and BiVO4 thin films were spin-coated on FTO in a intersecting way with the WO3 and BiVO4 overlapping only in the center part, resulting in multilayer fingers-crossing thin films (fingers-crossing heterojunction). Samples with only an overlapped area (sandwich heterojunction) were obtained by cutting off the non-overlapped area of both the WO3 and BiVO4 layers (the nodes) as a counterpart for comparison. The influence of layer number, layer thickness and junction type on the photoelectrochemical (PEC) water splitting performance under simulated solar light for the obtained heterojunctions was investigated. XRD, SEM, XPS and UV-vis absorption measurements were conducted to investigate the structural properties and morphology. The photocurrent density of the sandwich heterojunction is higher than that of the fingers-crossing heterojunction at a low applied bias. The IPCE shows that both the fingers-crossing heterojunction and sandwich heterojunction can effectively utilize light up to 510 nm. Mott–Schottky plots show that the sandwich heterojunction has a more negative flat band potential, which is desirable for water splitting, than the fingers-crossing heterojunction. In addition, the mechanism of photogenerated carrier separation for these two heterojunctions was discussed.

Comparison of sandwich and fingers-crossing type WO3/BiVO4 multilayer heterojunctions for photoelectrochemical water oxidation

Bismuth vanadate is a promising semiconductor for photoelectrochemical (PEC) water oxidation under visible light. Yet its PEC performance is limited by its poor electron-transfer mobility. In this study we modified the BiVO4 surface with Zn to address this problem. We used a ZnO coating and etching treatment to dope Zn atoms at the surface of BiVO4 by replacing the Bi atoms. The surface Zn doping treatment lowered the Fermi level of the outer part of BiVO4 (surface) and formed a BiVO4/Zn:BiVO4 homojunction with a type II band alignment with the inner part of BiVO4 (bulk). With this homojunction, the charge transfer process across the depletion region of the BiVO4 electrode was improved and the surface trapping of photogenerated electrons was avoided, which led to an enhanced PEC performance.

Enhanced Photoelectrochemical Performance of the BiVO4/Zn:BiVO4 Homojunction for Water Oxidation

BiVO4 is recognized as a promising semiconductor for photoelectrochemical (PEC) applications. However, the lack of synthesis methods to prepare high-quality nanostructural BiVO4 film is rarely reported in the PEC field. In this study, we report a novel synthesis approach to prepare one-dimensional BiVO4 nanowire arrays using a multistep ion exchange approach through a solvothermal–hydrothermal–annealing process. The resulting BiVO4 electrodes showed a nanorod structure with high porosity. In particular, the aspect ratio surface of the BiVO4 nanostructure was found favorable for its PEC application. The BiVO4 nanostructure with an optimized synthesized condition showed efficient PEC water oxidation with a photocurrent of 1.67 mA/cm2 at 1.83 V (vs RHE).

A Multistep Ion Exchange Approach for Fabrication of Porous BiVO4 Nanorod Arrays on Transparent Conductive Substrate

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